We bought an Otty mattress about 6 months ago.
So far, so good. Very happy with it, nice and firm, have definitely slept better since we've had it. We did an offer where you get two free memory foam pillows (worth about £110) when you buy the mattress - better deal than the £50 off they were offering.
I imagine all the different brands are much of a muchness - some friends have Simba and love it - so maybe the decisive factor would be price/finance/special offers - Otty are offering 2 free pillows or £50 off the price at the moment I think.
It's crazy how the vacuum packing reduces the size so much, but once open, it 'grows' (for want of a better word) to it's correct size and although they say it take 48 hours to fully settle, you can sleep on it on the first night. They also take your old mattress away (well, Otty do, but it costs £30).
There is a 100 night free trial, but to be honest, we forgot all about that as we were so happy with the purchase. So I can't vouch for how much/little hassle it would be to return it if you're not happy, but 3 months is pretty fair I'd say.